You gotta remember that 
means 
.
So, in this case, f(x) is [4x - 3], g(x) is [x^3 + 2x].
So that : f(x)-g(x) = [4x-3] - [x^3+ 2x] = -x^3 + 4x - 2x - 3 = -x^3 + 2x - 3
So, in this case, f(x) is [4x - 3], g(x) is [x^3 + 2x].
So that : f(x)-g(x) = [4x-3] - [x^3+ 2x] = -x^3 + 4x - 2x - 3 = -x^3 + 2x - 3